FRIDAY, Aug. 28 (HealthDay News) -- Every person who is infected with the H1N1 swine flu puts 1.5 other people at risk over the three days before coughing, fever and other symptoms appear.

That's the conclusion of a new study published in the journal Influenza and Other Respiratory Viruses.

Anyone showing early symptoms of the flu needs to contact their health-care provider immediately. In addition, anti-viral drugs will likely help slow transmission, the researchers said.

The currently circulating swine flu first appeared in the town of La Gloria in Veracruz state in Mexico apparently sometime in early March. By March 15, it had spread to Mexico City and, since then, to much of the world.

According to the latest World Health Organization (WHO) figures, more than 209,400 cases of swine flu have occurred globally, with at least 2,185 deaths. These numbers likely underestimate the outbreak, WHO noted.

The authors of the new study looked at Mexico City data on all suspected cases of H1N1 swine flu from April 15 to April 25, examining people they had been in contact with, and cross-referencing that information with the onset of symptoms, hospitalization and other factors.

Their conclusions: The 2009 H1N1 virus is spreading at a rate comparable to the 1957 and 1968 flu pandemics -- the most recent pandemics prior to this year's swine flu -- and to the SARS (Sudden Acute Respiratory Syndrome) outbreak, which surprised the world in 2003.

"Even if flu has a reproduction only a little bit above 1 [in this case 1.5], it has its effect because, in a susceptible population, it can start jumping from person to person within one to two days," said study co-author Dr. Nathaniel Hupert, co-director of Cornell University and Weill Cornell Medical College's joint Institute for Disease and Disaster Preparedness. He is also director of the Centers for Disease Control and Prevention's Preparedness Modeling Unit.

"What that means is, three days later, you've got an additional half a person infected. In three days, each of those new people have infected an additional half person, and it's like compound interest. It's the same calculation that lets you grow $1,000 into a $1 million 20 years later."

But health officials stress that the H1N1 swine flu produces relatively mild infections, much like the annual seasonal flu, and patients recover quickly. And some people, mostly older ones, seem to have some immunity to the virus.

However, doctors around the world are reporting a very severe form of the disease in young and otherwise healthy people. "In these patients, the virus directly infects the lung, causing severe respiratory failure," the WHO said. So, countries should anticipate a growing demand for treatment in intensive care units as they prepare for a second wave of the pandemic, the agency said, the Associated Press reported.

Dr. Peter Gross, chief medical officer at Hackensack University Medical Center in New Jersey, sees no reason for mass panic. "They've proven that the transmissibility is comparable to the seasonal flu and less than the horrendous 1918 pandemic," he said. Also key, he said, is that "the mortality is no worse than the seasonal flu and, if anything, might be slightly less."

Still, he and others agree that the potential for infection is significant.

"If each person were infecting less than one person, it would eventually die out on its own. If it was two people, the outbreak would cascade. If it was 10, it would be an explosive epidemic," said Dr. Dean Blumberg, associate professor of pediatric infectious diseases at University of California, Davis Children's Hospital. "The number they've come up with here is similar to what others have found, in the range of 1.3 to 1.4. The seasonal flu is about 1.3, so it's right in the range."

The 1918 pandemic was estimated at about 2 to 2.5, he said.

"In a sense, it's kind of reassuring in that it is highly transmissible but not exceptionally transmissible," Blumberg said. "On the other hand, we need to keep in mind that, unless you're someone who has already been infected with the swine flu, everybody in the world is susceptible to it. That's the scary part."

Given this vulnerability, Blumberg said, it makes sense that hospitals and communities are taking some extra precautions, such as using N-95 respirator masks.

Authorities have tried various strategies to mitigate the current outbreak, including closing schools, although federal officials in the United States are leaving that decision to local jurisdictions.

Recommended precautions for preventing the spread of swine flu include avoiding contact with other people if you are sick, coughing into your sleeve rather than your hand, and copious hand washing.

But the study authors said larger-scale measures may still be needed.

The study is the first completed by the new Preparedness Modeling Unit at the U.S. Centers for Disease Control and Prevention.

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